Inherited deficiency of adenosine deaminase (ADA) causes severe combined immunodeficiency disease (SCID) in infancy, as well as more insidious forms of immune dysfunction that develop during childhood and adolescence (late onset). The long term objectives of this laboratory have been to understand the biochemical and molecular basis for immunodeficiency, and to use this information in order to develop effective therapy. The aims of the present proposal are to evaluate the genetic basis of ADA deficiency in patients with both SCID and late onset ADA deficiency to better understand the mechanisms by which specific mutations affect the stability and activity of ADA in cells of the immune system, and the role of specific mutations in determining both disease severity and the response to restoration of ADA activity by enzyme replacement with polyethylene glycol-modified adenosine deaminase (PEG-ADA). The answers to these questions are equally relevant to understanding the response of patients to various methods of somatic cell ADA gene transduction. The specific aims of the present proposal are: l) To characterize the mutant ADA genes of immunodeficient patients under treatment with PEG-ADA, and to investigate the relationship of these mutations to the degrees of enzyme deficiency, metabolic abnormality, clinical severity, and response to therapy. 2) To investigate, both in vitro and in cultured cells, the use of new approach, using sequence-specific antisense oligonucleotides, to enhance normal ADA expression in patients with mutations that affect the splicing of ADA mRNA. We will also investigate the hypothesis that alternative splicing contributes to residual ADA activity in some patients with splicing defects. 3) To investigate the interaction between mutant ADA proteins with the CD26/ADA complexing protein, to determine whether this interaction might influence ADA folding and stability, and thereby determine residual ADA activity and clinical severity.